This application claims the priority of Japanese Patent Application No. 2000-245194 filed on Aug. 11, 2000, which is incorporated herein by reference.
1. Field of the Invention
The present invention relates to an objective lens and optical pickup apparatus for recording and reproducing signals, which are commonly usable for two or more kinds of optical recording media having thicknesses different from each other. In particular, the present invention relates to an objective lens which, when light beams for irradiating the optical recording media have wavelengths different from each other according to the kinds of optical recording media, can efficiently converge the respective light beams onto their corresponding optical recording media; and an optical pickup apparatus using the same.
2. Description of the Prior Art
In recent years, various kinds of optical recording media have been under development, and optical pickup apparatus which can carry out recording and reproducing while using a plurality of kinds of optical recording media in common have been known. For example, a system which carries out recording and reproducing of DVD (digital versatile disc) and CD-R (recordable optical disc) by using a single optical pickup apparatus has been known.
In such two kinds of optical recording media, for example, visible light at about 650 nm is used for DVD in order to improve the recording density, whereas near-infrared light at about 780 nm is required to be used for CD-R since it has no sensitivity for light in the visible region. An optical pickup apparatus which can be used in common for both of them is based on a dual-wavelength beam type which uses two light beams having wavelengths different from each other.
In the two optical recording media exemplified above, however, the disc thickness of the CD-R is fixed to 1.2 mm whereas the DVD in use mostly has a thickness of 0.6 mm. Therefore, the individual wavelengths of light for reproducing or recording are needed to be securely converged at their predetermined positions different from each other.
Under such circumstances, a system in which two objective lenses having converging actions different from each other are made switchable depending on the optical recording media subjected to reproducing or recording has been known. However, it complicates the structure of optical pickup apparatus and opposes the demands for compactness and lower cost.
On the other hand, it has recently been known to use a multifocal lens composed of a diffractive optical element or a plurality of circular strip divisions for an objective lens part of an optical pickup apparatus (Japanese Unexamined Patent Publication No. 8-62493, Japanese Unexamined Patent Publication No. 9-145994, and the like).
In the apparatus using the above-mentioned diffractive optical element, however, a noise light component of an order different from that contributing to the recording and reproducing is emitted from the diffractive optical element. In the apparatus using the above-mentioned multifocal lens, the light transmitted through a circular strip division other than that contributing to the recording and reproducing at that time is noise light. Due to these noise light components, the S/N ratio at the recording and reproducing deteriorates.
In view of the circumstances mentioned above, it is an object of the present invention to provide a compact, inexpensive objective lens with a favorable S/N ratio for optical recording media, which can favorably converge two wavelengths of light onto their corresponding recording surfaces of optical recording media having respective thicknesses without complicating the structure of optical pickup apparatus; and an optical pickup apparatus using the same.
The present invention provides an objective lens for optical recording media, the objective lens being constituted by two lenses cemented together and used for recording or reproducing two kinds of optical recording media having thicknesses different from each other with two light beams having wavelengths different from each other, respectively;
the objective lens satisfying the following conditional expression (1) with respect to a light beam having a wavelength xcex1 used for recording or reproducing one of the two optical recording media, and
the following conditional expression (2) with respect to a light beam having a wavelength xcex2 used for recording or reproducing the other of the two optical recording media:
NLxcex1 greater than NDxcex1xe2x80x83xe2x80x83(1)
NLxcex2 less than NDxcex2xe2x80x83xe2x80x83(2)
xe2x80x83where
NLxcex1 is the refractive index of the lens on the light source side with respect to the light beam having the wavelength xcex1;
NDxcex1 is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength xcex1;
NLxcex2 is the refractive index of the lens on the light source side with respect to the light beam having the wavelength xcex2; and
NDxcex2 is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength xcex2.
Also, the present invention provides an objective lens for optical recording media, the objective lens being constituted by two lenses cemented together and used for recording or reproducing two kinds of optical recording media having thicknesses different from each other with two light beams having wavelengths different from each other, respectively;
the objective lens satisfying the following conditional expression (3) with respect to a light beam having a wavelength xcex1 used for recording or reproducing one of the two optical recording media, and
the following conditional expression (4) with respect to a light beam having a wavelength xcex2 used for recording or reproducing the other of the two optical recording media:
NLxcex1 less than NDxcex1xe2x80x83xe2x80x83(3)
NLxcex2 greater than NDxcex2xe2x80x83xe2x80x83(4)
xe2x80x83where
NLxcex1 is the refractive index of the lens on the light source side with respect to the light beam having the wavelength xcex1;
NDxcex1 is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength xcex1;
NLxcex2 is the refractive index of the lens on the light source side with respect to the light beam having the wavelength xcex2; and
NDxcex2 is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength xcex2.
Further, the present invention provides an objective lens for optical recording media, the objective lens being constituted by two lenses cemented together and used for recording or reproducing two kinds of optical recording media having thicknesses different from each other with two light beams having wavelengths different from each other, respectively;
assuming that the optical recording medium recorded or reproduced with a light beam having a wavelength xcex1 and the optical recording medium recorded or reproduced with a light beam having a wavelength xcex2 have respective thicknesses t1 and t2 satisfying the condition of t1 less than t2,
the objective lens satisfying the following conditional expression (5) when the cemented face of the cemented lenses is convex toward the light source, and
the following conditional expression (6) when the cemented face of the cemented lenses is convex toward the optical recording media:
NLxcex1 greater than NDxcex1 and NLxcex2 less than NDxcex2xe2x80x83xe2x80x83(5)
xe2x80x83NLxcex1 less than NDxcex1 and NLxcex2 greater than NDxcex2xe2x80x83xe2x80x83(6)
xe2x80x83where
NLxcex1 is the refractive index of the lens on the light source side with respect to the light beam having the wavelength xcex1;
NDxcex2 is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength xcex1;
NLxcex2 is the refractive index of the lens on the light source side with respect to the light beam having the wavelength xcex2; and
NDxcex2 is the refractive index of the lens on the optical recording media side with respect to the light beam having the wavelength xcex2.
Furthermore, the present invention provides an objective lens for optical recording media, the objective lens being constituted by three lenses cemented together and used for recording or reproducing two kinds of optical recording media having thicknesses different from each other with two light beams having wavelengths different from each other, respectively;
assuming that the optical recording medium recorded or reproduced with a light beam having a wavelength xcex1 and the optical recording medium recorded or reproduced with a light beam having a wavelength xcex2 have respective thicknesses t1 and t2 satisfying the condition of t1 less than t2,
the objective lens satisfying the following conditional expression (7) when the cemented face between the first and second lenses from the light source side is convex toward the light source:
N1xcex1 greater than N2xcex1 and N1xcex2 less than N2xcex2xe2x80x83xe2x80x83(7)
the objective lens satisfying the following conditional expression (8) when the cemented face between the first and second lenses from the light source side is convex toward the optical recording media:
N1xcex1 less than N2xcex1 and N1xcex2 greater than N2xcex2xe2x80x83xe2x80x83(8)
the objective lens satisfying the following conditional expression (9) when the cemented face between the second and third lenses from the light source side is convex toward the light source:
N2xcex1 greater than N3xcex1 and N2xcex2 less than N3xcex2xe2x80x83xe2x80x83(9)
the objective lens satisfying the following conditional expression (10) when the cemented face between the second and third lenses from the light source side is convex toward the optical recording media:
N2xcex1 less than N3xcex1 and N2xcex2 greater than N3xcex2xe2x80x83xe2x80x83(10)
xe2x80x83where
N1xcex1 is the refractive index of the first lens from the light source side with respect to the light beam having the wavelength xcex1;
N2xcex1 is the refractive index of the second lens from the light source side with respect to the light beam having the wavelength xcex1;
N3xcex1 is the refractive index of the third lens from the light source side with respect to the light beam having the wavelength xcex1;
N1xcex2 is the refractive index of the first lens from the light source side with respect to the light beam having the wavelength xcex2;
N2xcex2 is the refractive index of the second lens from the light source side with respect to the light beam having the wavelength xcex2; and
N3xcex2 is the refractive index of the third lens from the light source side with respect to the light beam having the wavelength xcex2.
Preferably, the objective lens in this case satisfies the following conditional expressions (11) and (12):
N1xcex1=N3xcex1xe2x80x83xe2x80x83(11)
N1xcex2=N3xcex2xe2x80x83xe2x80x83(12)
Preferably, the first and third lenses from the light source side are formed from the same material.
The cemented faces of the lenses may be made aspheric.
In each of the above-mentioned objective lenses for optical recording media, one of the light beams having wavelengths xcex1 and xcex2 may be a light beam having a wavelength of 650 nm used for recording or reproducing DVD, whereas the other may be a light beam having a wavelength of 780 nm used for recording or reproducing CD-R.
The present invention provides an optical pickup apparatus comprising any of the above-mentioned objective lens for optical recording media.
In general, objective lenses for optical discs commonly used for recording/reproducing various kinds of discs having respective thicknesses different from each other have the following problems in general.
Namely, if an objective lens designed for a relatively thick disc is used for recording/reproducing a relatively thin disc, a negative spherical aberration will occur, whereby recording/reproducing cannot be effected sufficiently.
If an objective lens designed for a relatively thin disc is used for recording/reproducing a relatively thick disc, by contrast, a positive spherical aberration will occur, whereby recording/reproducing cannot be effected sufficiently.
For example, if an objective lens designed for CD-R (having a thickness of 1.2 mm) is used for DVD (having a thickness of 0.6 mm), a negative spherical aberration will occur, whereby recording/reproducing cannot be effected sufficiently.
If an objective lens designed for DVD (having a thickness of 0.6 mm) is used for CD-R (having a thickness of 1.2 mm), by contrast, a positive spherical aberration will occur, whereby recording/reproducing cannot be effected sufficiently.
Therefore, when a lens is to be designed such that both of the above-mentioned thick and thin discs can be recorded/reproduced, lens design parameters are required to be such that the spherical aberration changes in opposite directions depending on whether the above-mentioned thick or thin disc is used.
When a lens surface is convex onto the air side at the boundary between the lens and air, i.e., it is a lens surface having a positive refracting power, the spherical aberration will change in the negative direction as the curvature of the lens surface increases in general.
When the lens surface is concave onto the air side, i.e., it is a lens surface having a negative refracting power, by contrast, the spherical aberration will change in the positive direction as the curvature of the lens surface increases in general.
A lens constituted by a single group having a single element configuration has only boundary faces in contact with the air. Therefore, as can be seen from the foregoing, this lens has no lens surface which can change the spherical aberration in both of the positive and negative directions at the same time when the curvature of boundary faces is changed.
By contrast, a cemented lens can have either a positive or negative refracting power due to the difference in magnitude of refractive indices of materials in front and rear of the cemented face even when the surface form is the same.
Namely, when the cemented face is convex toward the light source, for example, if the lens material positioned on the light source side of the cemented face has a refractive index lower than that of the lens material positioned on the optical recording media side of the cemented face, then the cemented face can be formed as a lens surface having a positive refracting power. If the lens material positioned on the light source side of the cemented face has a refractive index higher than that of the lens material positioned on the optical recording media side of the cemented face, by contrast, then the cemented face can be formed as a lens surface having a negative refracting power.
Thus, the cemented face can be made as a lens surface having a positive refracting power or a lens surface having a negative refracting power according to the difference in refractive index on the front and rear sides thereof even when the surface form is the same. When the curvature of the cemented face is made large, the spherical aberration can be changed in the negative or positive direction according to the refracting power of the lens surface at this time.
Taking account of such a property of the cemented face, the present invention is achieved.
When the wavelength of light employed varies depending on the disc thickness, the wavelength-dependent difference in refractive index of the material constituting the lens due to the above-mentioned property of the cemented face is utilized so as to exhibit a negative refracting power (or positive refracting power) and a positive refracting power (or negative refracting power) with respect to the light beams having wavelengths xcex1 and xcex2, respectively.
For example, suppose a case where a cemented lens is used as an objective lens for an optical disc for recording/reproducing discs having thicknesses of t1 and t2 (t1 less than t2) with light beams having wavelengths xcex1 and xcex2, respectively, whereas its cemented face is convex toward the light source. In this case, if the lens material on the light source side of the cemented face has a refractive index higher than that of the lens material on the optical recording media side with respect to the light beam having the wavelength xcex1 and has a refractive index lower than that of the lens material on the optical recording media side with respect to the light beam having the wavelength xcex2, then the cemented face can exhibit negative and positive refracting powers with respect to the light beams having wavelengths xcex1 and xcex2, respectively.
As the curvature of the cemented face is enhanced, the spherical aberration can be changed in the positive direction with respect to the light beam having the wavelength xcex1, and in the negative direction with respect to the light beam having the wavelength xcex2. Thus, lens design parameters can exist such that the spherical aberration changes in opposite directions depending on whether a relatively thick or thin disc is used, thereby making it possible to construct an objective lens which can record/reproduce discs having thicknesses different from each other.
When the cemented face is convex toward the optical recording media in the above-mentioned example, effect similar to those mentioned above can be obtained if the relationship between the refractive indices of lens materials is in the reverse of that mentioned above.
Though Japanese Unexamined Patent Publication No. 10-332906 and Japanese Patent Publication No. 2727373 disclose cemented objective lenses for optical pickups, each of them is assumed to use a single wavelength of light, whereby they are hard to exhibit the effects of the present invention.